The present invention relates to implants for use in lumbar interbody fusion procedures and instruments for performing such procedure. More specifically, this invention relates to implants formed from donor bone that are useful to restore disc height and promote bone fusion after discectomy and to instruments and methods for preparing the intervertebral space and inserting the implant into an intervertebral space.
One of the leading causes of lower back pain and disability results from the rupture or degeneration of one or more lumbar discs in the spine. Pain and instability are caused by compression of the spinal nerve roots because the damaged discs protrude into the vertebral canal and do not provide sufficient biomechanical support for the full range of vertebral motion. Normally intervertebral discs, which are located between endplates of adjacent vertebrae, stabilize the spine and distribute forces between the vertebrae and cushion vertebral bodies. An intervertebral disc includes a semi-gelatinous componentxe2x80x94the nucleus pulposus, and a fibrous ringxe2x80x94the annulus fibrosis. The spinal discs may be displaced or damaged due to trauma, disease or aging. A herniated or ruptured annulus fibrosis may result in nerve damage, pain, numbness, muscle weakness, and even paralysis. Furthermore, as a result of the normal aging processes, discs dehydrate and harden, thereby reducing the disc space height and producing instability of the spine and decreased mobility.
Not all patients with damage or spinal deformities require surgical intervention. However, patients who have failed to respond to conservative treatment and who have demonstrable disc pathology often require surgical correction. Most typically the surgical correction includes a discectomy (surgical removal of a portion or all of the intervertebral disc). Discectomy is often followed by fusion of the adjacent vertebrae. To alleviate the pain, abnormal Joint mechanics, premature development of arthritis, and nerve damage, the disc space vacated by the damaged disc must be preserved following discectomy. Therefore, spacers or implants are required between the vertebrae that were adjacent to the resected disc.
Current treatment methods have been unable to accurately control the endplate removal using conventionally designed chisels, scrappers and cutters. Use of conventional surgical instruments does not adequately control the depth of cut into the disc space nor provide a means to accurately the countersink the implant into the prepared cavity-particularly for non threaded impacted. Furthermore, current methodologies do not provide sufficient protection of the neural structures during surgery to prevent neural injury
Current treatment methods utilize grafts, either bone or artificial implants, to fill the intervertebral space between adjacent vertebrae. It is desirable that these implants not only fill the disc space vacated by the damaged disc, but also restore the disc space height to pre-damaged condition. An implant must be sufficiently strong to bear substantially all the body""s weight above the vertebral space where it is inserted. Furthermore, it is desirable to use the implants to promote fusion of the adjacent vertebrae across the disc space and thereby promote mechanical stability. Current methodologies use implants or spacers made of metal, plastic composites or bone. Use of bone implants offers several advantages over artificial spacers or implants. The bones provide an implant having a suitable modulus of elasticity that is comparable to that of the adjacent vertebrae. The bone implants can be provided with voids, which can be packed with cancellous bone or other osteogenic material to promote bone growth and eventual fusion between adjacent vertebrae. Furthermore, implants formed from cortical bone have sufficient compressive strength to provide a biomechanically sound intervertebral spacer while it is slowly being incorporated or absorbed by the body and substituted for the patient""s own bone tissuexe2x80x94colloquially referred to as xe2x80x9ccreeping substitution.xe2x80x9d
While it is desirable to use natural bone grafts as implants, use of bone is often limited because of a small supply of suitable sources. Xenografts from non-humans, animals, suffer from rejection problems once implanted. While measures are being taken to limit the human body""s rejection of xenografts, greater success is still achieved with bone obtained from human sources. The best source is an autograft from the patient receiving the graft. Removal of an autograft requires further surgery and is limited in amount and structural integrity by the patient""s anatomy. The alternative source of human bone grafts is allografts harvested from human donors. Since the number of people donating tissue to science is small, these bone grafts represent an extremely valuable and rare commodity. Current methodologies for providing cortical bone implant spacers typically require cutting the spacer, usually in the form of a dowel, from the diaphysis of a long bone. Only a certain portion of the diaphysis is sufficiently thick to provide dowels with requisite strength to maintain the intervertebral space. For example, in a human femur only about the middle third of the diaphysis, where the shaft is narrowest and the medullary canal is well formed, has sufficient bone wall thickness and density to be used to prepare cortical dowels. The suitable portions of the diaphysis are sliced, and then a plug is cut from each slice. The plugs are then machined to form a dowel. Most often the dowel includes the medullary canal to provide a depot for osteogenic material and promote fusion of the adjacent vertebrae. Much of the donor bone is wasted, particularly the remnants of the slices from the diaphysis used to provide the dowels as well as the end portions of the long bone which cannot be utilized. Above and below this middle section of the diaphysis, the walls of the femur bone become thinner because of the separation of the layers of the bone into cancelli. Thus, these portions of the femur are not considered suitable for forming cortical dowels having the required dimensions for inserting into vertebral spaces. Use of these remnants would provide a more efficient use and conservation of a limited and very valuable natural resource of cortical bone.
Thus, there remains a need for improved bone graft implants and instruments for their placement in the body.
Thus, there is provided with the present invention an implant prepared from bone. The implant has a shaped body and sufficient length to extend from an anterior portion of a vertebral body to a posterior portion and a height sufficient to separate adjacent vertebrae. The implant comprises a bone portion having an upper and lower bone engaging surface, a first sidewall and an opposite second sidewall. The first and second sidewalls extend between the upper and lower bone engaging surfaces. Furthermore, the first sidewall includes a portion defined by a concave surface. Preferably, the implant also includes a recessed region that serves as a depot or receptacle for deposition of osteogenic material to enhance bone growth and eventual fusion of the adjacent vertebrae. One end of the implant is provided with structural features to engage an implant holder; in a preferred embodiment, the other end of the implant is provided with a shape to ease insertion into the intervertebral space. The implant prepared according to the present invention has sufficient biomechanical support to maintain the desired intervertebral space while it is gradually being replaced with new bone growth. In specific embodiments of the present invention, the implant is provided in the form of a J-shape; in other specific embodiments the implant is provided in the form of a crescent shape.
There is also provided in accordance with the present invention an implant holder. The implant holder includes a gripping head for releasably securing the implant. Preferably the impact holder also includes an impacting surface for driving the implant into a preformed cavity in the intervertebral space. The gripping head includes at least one implant engaging structural feature, such as a pin. Optimally, the pin includes at least a radiopaque portion to provide a means for viewing placement of the implant via radiography during surgery. In one embodiment, the gripping head on the implant includes a surface preferably roughened or knurled to secure the implant into the preformed cavity. In another embodiment of the present invention, the implant holder including the gripping head includes at least two surfaces. Each of the surfaces includes a pin that releasably secures the implant to the gripping head by matingly engaging recesses in the implant. The implant holder of the present invention securely holds the implant so that it can be impacted into the preformed cavity in the intervertebral space and then releases the implant so it remains in the cavity.
There is also provided in the present invention instruments for preparing an intervertebral space for receiving the final fusion implant. The instruments include a chisel for preparing the intervertebral space to receive the implant, preferably by cutting a cavity in the opposing endplates of adjacent vertebrae. The bone chisel includes at least two cutting edges. The bone chisel also includes opposing curved surfaces extending distally beyond the cutting surface to center the chisel and the cutting edges between the opposing surfaces of adjacent vertebrae. When the chisel and cutting blades are centered between the opposing surfaces, the blades cut equally from both surfaces.
Other instruments included for use in the present invention include scrapers, rotating scrapers, and impacting or xe2x80x9cslapxe2x80x9d hammers for driving the implants into position. The slap hammer allows for controlled impacting force and removal of the chisel after cutting.
Another aspect of the present invention includes a nerve retractor blade assembly for manipulation of neural structures such as the dural sac and traversing nerve root with minimal trauma to the respective structures. The assembly includes a retractor having a channel adapted to receive a retractor blade, a retractor blade and can include at least one pin, preferably two pins, for securely fixing the positioning of the retractor assembly and blade proximal to the intervertebral space. The blade can be adapted for engagement with the retractor and for extending into the intervertebral space to provide anchorage proximal to the disc space, maintain disc height and maintain distraction. The nerve retractor also includes a handle attached at an angle to the channel. In a preferred embodiment, the retractor channel is provided in the form of a concave channel. It is also contemplated that the retractor channel can be formed in a variety of other shapes, for instance, rectangular and broadened V-shaped channels are also included within the present invention.
Yet another aspect of the present invention includes a protective guide sleeve. The protective sleeve includes a body having a hollow core for receiving instruments for performing surgery. In addition, the protective sleeve provides a narrow but unobstructed passageway to the intervertebral space and minimizes the area of tissue impacted by the surgery. Preferably the protective sleeve includes a first and a second distractor fin that can be inserted into the intervertebral space to maintain the space height and alignment of the vertebrae. The protective sleeve provides protection for neural structures and prevents encroachment of the neural structures into the surgical area. The protective sleeve allows use of depth stop on surgical tools. Preferably, the protective sleeve includes at least one window to facilitate visualization. The instruments for preparing and inserting spinal fusion implants can be received within the protective sleeve. Such instruments include the implant holder engaged to an implant, the bone chisel, scrapers, and drills provided in the present invention.
Accordingly, it is one object of the present invention to provide an improved implant of bone to maintain an intervertebral space after discectomy.
It is another object of the present invention to provide an implant holder to releasably secure the implant and facilitate impaction of the implant into position within the intervertebral space.
It is still another object of the present invention to provide a cutting chisel to prepare a cavity in the intervertebral space between two opposing adjacent endplates on vertebrae.
It is yet another object of the present invention to provide a retractor for manipulation of neural structures.